1. Field of the Invention
This invention relates to a method of avoiding excessive agglomeration of carbonaceous solid particles so as to prevent defluidization in a fluid-bed reaction zone. More particularly, it is an improved method for injecting fresh carbonaceous particles into a fluid-bed hydrocarbonization, gasification or carbonization reaction zone.
2. Description of the Prior Art
Increasing energy needs have focused attention on solid fossil fuels due to their availability in the United States in a relatively abundant supply and their potential value if converted into more useful forms of energy and feedstock. Processes such as carbonization, gasification, hydrocarbonization and hydrogasification, wherein synthetic fuel products have been prepared by introducing a fluidized stream of finely-divided coal or other solid carbonaceous particles into a fluid-bed reaction zone and reacting the said particles at elevated temperatures in the presence of air, steam, hydrogen or inert gases are well known. A major operating difficulty in such processes has been the tendency of coal or other carbonaceous particles, especially intensified in a hydrogen-rich atmosphere, to agglomerate at the elevated temperature required for reaction.
Coal particles, especially caking, swelling or agglomerating coals, become sticky when heated in a hydrogen-rich atmosphere. Even non-caking, non-swelling and non-agglomerating coals become sticky when heated in such an atmosphere. Coal particles begin to become sticky at temperatures in the range of from about 280.degree. C., commonly from about 350.degree. C. to about 500.degree. C., depending on the specific properties of the coal, the atmosphere and the rate of heating. Such stickiness is due to a tarry or plastic-like material forming at or near the surface of each coal particle, by a partial melting or decomposition process. On further heating over a period of time, the tarry or plastic-like material is further transformed into volatile products and a substantially porous, solid material referred to as a "char." The length of this time period depends upon the actual temperature of heating and is shorter with an increase in temperature. The term "plastic transformation" as used herein refers to such tendency of the surfaces of coal or other carbonaceous particles being heated, particularly when heated in a hydrogen atmosphere, to develop stickiness and transform into substantially solid char, non-sticky surfaces. "Plastic transformation" is undergone by both normally agglomerating coals and coals which may develop a sticky surface only in a hydrogen-rich atmosphere.
Agglomerating or caking coals partially soften and become sticky when heated to temperatures between about 280.degree. C., commonly from about 350.degree. C., to about 500.degree. C. The duration of stickiness depends on the temperature of the coal, being on the order of minutes at the lower end of said range and being exponentially shorter, i.e. down to seconds, at the upper limits of said range. Components of the coal particles soften and gas evolves because of decomposition. Sticky coal particles undergoing plastic transformation tend to adhere to most surfaces which they contact such as walls or baffles in the reactor, particularly relatively cool walls or baffles. Moreover, contact with other sticky particles while undergoing plastic transformation results in gross particle growth through adherence of sticky particles to one another. The formation and growth of these agglomerates interferes drastically with the maintenance of a fluid-bed and excessive growth can make it impossible to maintain fluidization.
In particular, entrance ports and gas distribution plates of equipment used in fluid-bed coal conversion processes become plugged or partially plugged. Furthermore, even if plugging is not extensive, the sticky particles tend to adhere to the walls of the reaction vessel, with continued gross particle growth and the formation of multi-particle conglomerates and bridges interfering with smooth operation and frequently resulting in complete stoppage of operation as a result of defluidization of the bed.
Agglomeration of coal particles upon heating depends on operating conditions such as the heating rate, final temperature attained, ambient gas composition, coal type, particle size and total pressure. Even non-agglomerating coals, such as lignites or coals from certain sub-bituminous seams, are susceptible to agglomeration and tend to become sticky when heated in a hydrogen atmosphere. Thus, agglomeration of coal particles is accentuated in a hydrocarbonization reactor where heating in the presence of a hydrogen-rich gas actually promotes formation of a sticky surface on the coal particles reacted. Introducing any carbonaceous, combustible, solid particles, even those normally non-agglomerating, to a fluid-bed having an atmosphere tending to induce agglomeration can, moreover, result in agglomeration and defluidization of the bed.
Heavy liquid materials are also fed at times to the fluid-bed in coal conversion processes. They may be recycled heavy tar products to be converted to lower molecular weight products, light liquids and gases. Or they may be heavy liquids added from an external source to, for example, enrich the normal gas and/or liquid product, or as a means of waste disposal. Feeding such liquids is known to cause rapid loss of fluidization due to excessive particle agglomeration and plugging.
In an attempt to overcome the problems associated with agglomeration, char as a recycle material from fluidized bed processes has been mixed with an agglomerating type coal feed at a ratio as high as 8 to 1. Also, tar has been ball-milled with a great excess of absorbent char before feeding into a fluid-bed reaction zone. Such procedures reduce the unit throughput, are wasteful of energy and are, therefore, costly. Other attempts have included a pretreatment step wherein coal is oxidized and/or devolatilized superficially in order to prevent sticking and agglomeration of particles, but this lowers the yield of useful products and adds to the overall cost of the operation. Thus, it is highly desirable economically to avoid or at least reduce the extent to which such oxidation pretreatment or such char recycle is employed.
An alternate approach is that suggested by Knudsen et al, U.S. Pat. No. 3,927,996, in which the fines carried overhead by gas from a fluid-bed are monitored and the injection velocity of fresh feed material is regulated in response to changes in the fines content of the gas to produce controlled attrition of agglomerated particles in the fluid-bed. In this approach, a caking coal or other similar carbonaceous solid is introduced into a fluidized bed containing char particles maintained at a temperature in excess of the coal resolidification point by entraining coal particles in a gas stream preheated to a temperature in excess of about 300.degree. F., i.e. about 150.degree. C., but below the initial softening point of the coal. For the gasification of bituminous coals, preheat temperatures up to about 550.degree. F., i.e. about 285.degree. C., are said to be preferred. A fluid-cooled nozzle 16 is employed for feeding the stream of carrier gas and entrained coal particles into the gasifier zone. The injection velocity is regulated between superficial gas velocities as low as 15 feet/second and as high as 1,000 feet/second in response to variations in the fines content of the overhead gas. Such a system necessarily requires continual processing adjustments that are not desirable in continuous, commercial scale operations. In addition, the intermittent high injection velocities of the fresh coal introduced into the fluid-bed under the indicated conditions would generally be considered as having a potential for injection nozzle erosion that, if severe, could lead to a need for premature shutdown for nozzle replacement, adversely affecting the overall effectiveness of the coal conversion operation being carried out in the fluid-bed reaction zone.
A need thus exists in the art for improved methods for treating agglomerating coal or other solid carbonaceous particles in fluid-bed reaction zones. This need resides with respect to the effective injection of fresh particles of such coal or other carbonaceous materials under conveniently controlable conditions capable of avoiding excessive agglomeration of feed particles and thus preventing defluidization of the bed. Such improved methods would desirably avoid the necessity for pretreatment oxidation of the feed particles and/or their admixture with recycle char particles prior to being introduced into the fluid-bed reaction zone. The improvements required for technically and economically feasible coal injection operations must not, on the other hand, introduce peripheral processing disadvantages, such as undue injection nozzle wear, that would adversely affect the overall coal or other solid carbonaceous particle conversion operation.
It is an object of the invention, therefore, to provide a method of preventing excessive agglomeration of carbonaceous feed material in fluid-bed conversion operations.
It is another object of the invention to provide a method of avoiding defluidization in fluid-bed reaction zones employed in coal or other solid carbonaceous conversion operations.
It is another object of the invention to provide a method for employing caking coals on a continuous basis in a continuous fluid-bed reaction zone without defluidization and/or undue equipment plugging problems.
It is a further object of the invention to provide a method for avoiding excessive feed particle agglomeration while, at the same time, avoiding undue injection nozzle erosion.
It is a further object of the invention to provide improvements in the hydrocarbonization process for the preparation of fuel products from coal.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.